The US military has lifted its ban on flights of Osprey tilt-rotor transport aircraft, which had been grounded after a fatal crash in southwestern Japan in November.

The military announced the decision on Friday.

A CV-22 Osprey of the US Air Force crashed into the sea off Yakushima Island in Kagoshima Prefecture on November 29. All eight crewmembers on board were killed.

In December, the US military grounded not just its CV-22 fleet used by the Air Force but all Ospreys deployed globally, including the MV-22s used by the US Marine Corps and others.

The US military says the timing of resuming Osprey operations will be determined in line with procedures necessary in individual forces, such as the Air Force and the Marine Corps.

A senior official of the US forces told reporters that the decision was made based on "a meticulous and data-driven approach prioritizing the safety" of Osprey crews.

The official cited a material failure of an aircraft component as the cause of the November accident, but said they have not been able to find out what caused the problems. The official did not identify which component failed, saying that there is an inherent risk to what they do in military operations.

As measures to ensure the safety of flights, the military plans to change procedures for inspections and maintenance, but has no plans to replace equipment or parts.